More users and more data-intensive uses are hosted by cellular radio systems such as for example 3G and 4G radio access technologies (RATs). It is anticipated that continued efficiency improvements will not be able to keep up with the increase in data throughput and so there has been recent research into exploiting license exempt bandwidth to meet the increasing demand. Commonly the license exempt bandwidth is referred to as the Industrial, Scientific and Medical (ISM) band and/or television whitespaces (TV WS) or more generically the shared band(s), though these are not exhaustive.
The wireless local area network (WLAN) family of standards (commonly the RATs specified by IEEE 802.11b/g/n) typically use an access point (AP) which aids in coordinating access of the ISM band by individual users to minimize interference which is wasteful of the limited bandwidth. Different nations define the ISM band a bit differently but for WLAN the 2.4 GHz ISM band is generally divided into multiple overlapped channels. For example, in China and Europe (excluding Spain and France) the 2.4˜2.4835 GHz band is divided into 13 channels each with a bandwidth of 22 MHz, which support 3 orthogonal channels at the same time.
Cellular network operators (licensed band) in many regions of the world are seeking to offload some of their traffic to the license exempt band by setting up their own WLAN Aps, particularly in cities where data usage is densely packed. But to successfully offload traffic from the licensed to the license exempt band there needs to be some coordination or common understanding so that all users, cellular offload as well as conventional WLAN stations (STAs), can efficiently access the channel without undue interference to one another.